An internal combustion engine that is provided as a generator drive is conventionally supplied to the end user without a clutch and generator. The clutch and the generator are only mounted at the end customer's location. In order to ensure a consistent rated frequency for power supply into the network, the internal combustion engine is operated in a speed control loop. This detects the speed of the crankshaft as a controlled variable and compares it to a target engine speed, i.e., the reference variable. The resulting control deviation is converted by means of a speed regulator into a manipulated variable for the internal combustion engine, for example, an injection quantity. The problem with such a control loop is that torsional oscillations, which are superimposed on the controlled variable, can be reinforced by the speed regulator. Particularly critical are the low-pass oscillations caused by the internal combustion engine, for example, torsional vibrations of the 0.5 and 1st order. When starting the drive system the amplitudes of the torsional oscillations can become so large due to reinforcement by the speed regulator that a limit speed is exceeded, and the internal combustion engine shuts off.
The instability problem is countered with a speed filter in the feedback path of the speed control loop. We are familiar with such a speed filter from EP 0 059 585 B1. There the tooth timing values of a shaft are detected by means of a working cycle of the internal combustion engine. The working cycle includes two revolutions of the crankshaft, corresponding to 720 degrees. These tooth timing values are then used to calculate a filtered tooth timing value by forming an arithmetic mean. It is updated after every working cycle. This filtered tooth timing value corresponds to the actual speed value, which is then used to regulate the internal combustion engine. The problem with this 2-revolution filter, however, is that a stable behavior of the drive system produces a worsening of the load acceptance behavior.